Hitherto blow molding technique has been used for development of foamed blow-molded articles having a foam layer. Various methods have been thus proposed for production of foamed blow-molded articles. For example, there is known a method which includes melting and kneading a base resin and a blowing agent in an extruder, extruding the molten mixture through a die to obtain a foamed parison, sandwiching the foamed parison between molds, and subjecting the parison to blow molding.
Foamed blow-molded articles obtained by such a method are excellent in heat insulation property and light weight and, therefore, may be utilized for applications, such as automobile parts, receptacles, ducts and electric appliance parts, that require these properties. Especially, because hollow foamed blow-molded articles of a polypropylene-based resin have excellent heat resistance and excellent balance between their lightness in weight and rigidity, they are expected to be utilized as air conditioner ducts for automobiles.
However, a general polypropylene-based resin, which is a straight-chained crystalline resin and which undergoes a significant change in its melt properties at its foaming temperatures, is difficult in itself to expand to form a low apparent density molding. Even when a foamed parison is obtained by extrusion foaming of such a general polypropylene-based resin, it is still difficult to blow-mold the foamed parison into a complicated shape. In this circumstance, when a polypropylene-based resin is used as a base resin, obtained is only a foamed blow-molded article having a simple shape and an extremely low expansion ratio.
With a view toward solving the above problem, the present applicant proposes a method for producing a foamed blow-molded article using, as a base resin, a high melt tension polypropylene-based resin (HMSPP) having a specific melt tension and a specific melt flow rate (Patent Document 1). With this technique, it becomes possible to produce a polypropylene-based resin foamed blow-molded article having a wide range of apparent density. Among HMSPPs, those which have a branched structure show especially excellent foamability and, additionally, a foamed parison obtained therefrom shows excellent blow moldability.
In a blow molding process, since a parison is sandwiched and blow-molded between molds, a parting line is generally formed as a result of nipping and pressing by the molds on a periphery of the molded article with protruding fins being formed along the parting line in a large amount. Such fins are removed to obtain a final blow-molded article. The removed fins are generally pulverized and optionally further repelletized for use as a recycled raw material. Thus, the recycled raw material is used as a part of the raw material for the production of blow-molded articles.
When HMSPP having a branched structure is used as a raw material in blow molding, however, the viscoelastic property thereof tends to change due to cleavage of branched chains which is caused by heat and shear. As a consequence, the melt properties such as melt tension are apt to be deteriorated. Thus, depending upon the extrusion conditions, there is caused a case in which the melt tension and melt flow rate of HMSPP no longer meet the above-described specific requirements after it has been extruded. Therefore, when the melt properties of a recycled raw material are significantly deteriorated and when the blending amount of the recycled raw material is excessively large, the foamability and moldability are deteriorated so that it is not easy to obtain a foamed blow-molded article having a complicated shape and a low apparent density. Thus, there is a room for improvement in this respect.
The present applicant further proposes a technique for obtaining a foamed molded article having a high expansion ratio and a beautiful appearance by using a mixture of HMSPP, a recycled HMSPP raw material having specific melt properties and a general polypropylene-based resin in a specific blending ratio (Patent Document 2).
Additionally, the present applicant proposes a technique for producing a foamed blow-molded article having a thickness of 1-5 mm, a density of 0.35-0.65 g/cm3 and a uniform thickness by using a polypropylene-based resin having a specific equilibrium compliance and a specific swell, such as a mixture of HMSPP with its recycled raw material or a recycled raw material of HMSPP (Patent Document 3).
The production technique disclosed in Patent Documents 2 and 3 makes it possible to use an HMSPP recycled raw material for producing polypropylene-based resin foamed blow-molded articles.